The Quest for Perpetual Flight

In the middle of the night on June 1st, Alan Cocconi's "pilots" were holed up in a little 5×8-ft trailer in the California desert, glancing over each other's shoulders at the virtual gages on three computer screens, trying to keep their airplane aloft. Their task, even for instrument-rated flyers, was a formidable one: Maintain altitude by navigating from the ground, while looking only at telemetry data. Moreover, there was a power dilemma: The pilots were supposed to keep the plane flying on a trickle of electrical current from the on-board lithium-ion batteries, fueled only by energy from the sun that they had stored a few hours earlier.

Still, Cocconi's pilots kept the plane in the air, not just for that night, but for 48 consecutive hours. To date, their achievement is a record for a solar-powered plane—one considered so remarkable that it has begun raising hopes for a slew of new aerospace electronics applications ranging from low-orbiting geostationary satellites to battlefield communication centers.

"The building blocks that Alan demonstrated are applicable to a whole class of high-altitude, long-endurance aircraft," notes Stuart Hindle, vice president of strategy and business development for Cocconi's nearest competitor, AeroVironment, Inc. "A system like his could enable a multi-month operating aircraft in the stratosphere."

While the benefits of such systems might seem like a head-scratcher to most of us, Hindle and others are all-too-aware of their potential. Long-endurance aircraft, they say, are poised to change the face of communications, possibly serving as radio towers in the sky. Potential applications include wildfire detection, and hurricane and storm tracking, as well as reconnaissance and surveillance for battlefields and national security applications. Moreover, broadcast and telecom companies see tremendous potential in such technologies because these stratospheric airplanes could hover 2,000 times closer to earth than geostationary satellites, thus enabling them to serve as communication relays with approximately a thousand times more bandwidth density than satellites can provide.

"On a dollar-per-bit-per-second basis, you could do this for a fraction of the cost of a satellite," Hindle says. "The potential advantages are huge."

Powered by solar cells, Alan Cocconi's SoLong remained aloft for two days. Some engineers are hopeful that aircraft like SoLong could be used as "radio towers in the sky."(SoLong incorporates 76 solar cells, each measuring about 125x125 mm, across its 5-m wingspan.)

Focus on Efficiency

Indeed, the advantages of such aircraft are enormous, which begs the question: Why aren't they already dotting the horizon? The answer is that any such long-endurance aircraft, even one at an altitude of 70,000 ft, needs to make exceptionally judicious use of its fuel in order to fight the atmosphere while it stays in a fixed position above the earth.

That's where Cocconi comes in. Cocconi, founder, chairman, and chief engineer of AC Propulsion Inc. (www.acpropulsion.com), had long wanted to put his unusual combination of technical skills to work on a project that would enable an aircraft to remain aloft for days, or even months, at a time. A Cal Tech graduate with expertise in power electronics, as well as critical experience with General Motor's solar-power SunRaycer and a 30-year resume as a model airplane hobbyist, Cocconi seemed the ideal candidate for such a project. So early in 2003, he took a temporary leave from his job, dipped into his personal savings, and set to work on the aircraft that would eventually be dubbed "SoLong."

Even for the most determined optimist, however, SoLong's success seemed a tall order, especially since Cocconi was going it alone, and with a limited budget. The aircraft design called for Cocconi to supply expertise in four primary areas: composite structures; motor drives; autopilot systems; and silicon solar cells.

"I realized I was in a position to do this because of the things I had learned in electric car work, power electronics, and motors," Cocconi recalls. "It helped that I also had experience with airplane autopilots and control systems."

With that experience behind him, Cocconi initially built a model using a 5-m commercial wing, and no solar panels. Instead of drawing power from sunlight, he incorporated a large battery pack, and combined it with electric motors and custom-designed special motor drives. Cocconi flew that plane for almost two years—until early 2005—before beginning work on the solar-powered SoLong.

When he launched his effort on the solar-powered version, Cocconi's earlier work proved valuable, especially as he journeyed into the difficult area of power budgeting for a solar-powered aircraft. Unlike the original aircraft that he had flown for nearly two years, SoLong, he found, would need more sophisticated electronic drives to control the seven motors that powered its control surfaces and its propeller. The reason for the advanced drives was simple: SoLong required relatively high power (about 1000W) to take off, and yet needed high efficiency at low power (about 100W), so it could store sunlight energy while it cruised.

"The motor had to be very efficient at light throttle settings," Cocconi says. "That's not easy to achieve. High power and optimized efficiency at low power don't usually go together."

SoLong's ironless motor, combined with a drive custom-designed by Cocconi, provides the efficiency needed to keep the plane aloft indefinitely.

Without that efficiency, SoLong would have used all its energy to drive its propeller during the day, and therefore would have had none left for night operation. For that reason, Cocconi purchased high-efficiency, brushless, ironless motors from Kontronik Electric Power Systems (http://www.kontronikusa.com) and then developed his own specialized drives for those motors. Cocconi says he needed to develop the advanced drives because the motor's ironless design, when combined with a conventional pulsewidth-modulated drive, would have caused a large "ripple current," and therefore, would have reduced system efficiency. To circumvent the problem, Cocconi created a nine-phase drive using his own 2×1-inch driver board in conjunction with an 8-bit processor from Microchip Technology, Inc. (www.microchip.com). The split-phase drive produced a three-phase output with smoother current waveforms, very low ripple current, and greater efficiencies.

"The real advantage is that the motor now has 90 percent efficiency at 10 percent load," Cocconi explains. "And that's normally very hard to get with these motors."

Flying By Sunlight

While Cocconi optimized the high efficiency motors for SoLong, he also simultaneously developed the solar wing for his airplane, employing commercial-grade silicon solar cells for the wings, instead of the far more costly space-grade solar cells.

The flat solar cells were integrated into a "composite sandwich" wing made from carbon, Kevlar, and glass epoxy, and the wing was later flexed into an airfoil shape and then cured.

"To get space-grade cells, even space-grade rejects, would have raised the cost by a factor of ten," he says, "We couldn't afford to do that."

In all, Cocconi used 76 solar cells from SunPower Corp. (www.sunpowercorp.com), laminating them in place using CNC-machined aluminum molds. The flat cells were integrated into a "composite sandwich" wing made from carbon, Kevlar, and glass epoxy, and the wing was later flexed into an airfoil shape and then cured. The result was a solar wing containing silicon cells that conformed to the curvature of the wing.

In the end, Cocconi says, the choice of the commercial-grade SunPower cells was significant, because those cells incorporated critical rear-side electrical contacts. "When we laminated them onto the wing skins, all the contacts were in back," he recalls. "So the front was a very nice, smooth layer of fiberglass."

Cocconi laminated the solar cells in place using CNC-machined aluminum molds.

During its record-setting flight, SoLong used that solar wing in the daytime to convert sunlight energy into electrical current. Each cell produced about 3W of power, thus creating peak power of about 228W. Approximately 90 percent of that energy was stored in a 13-pound package of lithium-ion batteries (about 120 batteries in all).

Perpetual Flight

Having proven that the technology can keep an airplane aloft for two days and possibly longer, Cocconi is talking with engineers at Solar Impulse SA in Switzerland about a bigger project: a manned solar airplane that would circle the globe. The round-the-world flight, he says, could still be five years away, however,

Still, Cocconi and others see a place for such technologies in the near future, especially in communications applications. "That's always been the Holy Grail for solar-powered airplanes," he says. "People have been dreaming about it for more than 20 years."

Making the "radio tower in the sky" happen, however, won't be easy, experts say. Such aircraft would require payload capacities of 200 lbs or more for communications gear, and would need an extra kilowatt of power just to operate that equipment. As a result, a solar-powered aircraft could grow from Cocconi's 5-m wingspan of today to about 100 ft. And that, in turn, would substantially raise costs.

SoLong's pilots operated the aircraft from the team's ground-based 5x8-ft trailer in the Californina desert.

Once the technology has been proven on a commercial basis, however, many believe that telecom companies will be willing to pay high premiums for it.

Indeed, Cocconi and others in the long-endurance flight community believe that SoLong's two-day flight may have actually just scratched the surface of the technology's capabilities.

"Theoretically, that kind of system can operate forever," notes Hindle of AeroVironment. "Even from a practical standpoint, it might go six months between landings."

"SoLong was on a sustaining path," Cocconi adds. "With more pilots, it could have easily gone five days, and probably a lot longer than that."

To make it through the night, SoLong needed to store about 90 percent of the energy it collected during the day. Its nighttime power budget was as follows:

Motor power used at night

= 90 W × 10 hrs

= 900 W-hr

Transmitter, camera power used

= 7 W × 10 hrs

= 70 W-hr

Total power used during 10 night hours

970 W-hr (0.97 kW-hr)

Power stored during the day

= 1.20 kW-hr - 1.40 kW-hr

Remaining power

=0.2 - 0.4 kW-hr

Making Motors More Efficient

SoLong's ironless motor design, when combined with a conventional pulsewidth-modulated drive, would have caused a large "ripple current," and therefore, would have reduced system efficiency (top). Alan Cocconi's split-phase drive solved the problem by producing a three-phase output with smoother current waveforms, very low ripple current, and greater efficiencies (bottom).

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